As is known, a photoconductive surface in an electrophotographic printer is first charged to a uniform potential and then is "exposed" to an image to be reproduced by the scanning of a laser beam thereacross. The photoconductor thereby obtains an electrostatic latent image which, in a preferred embodiment, constitutes a matrix of discharged pixels on the photoconductor's surface. In a black/white printer, the photoconductive surface is developed using a black toner which adheres to the discharged pixel areas to form the image. Thereafter, the toned photoconductive surface is then carried to a transfer station where the image is transferred to a media sheet.
In a multi-color printer, successive images are developed employing different color toners supplied from corresponding toner modules. Color printing is normally done with yellow, cyan and magenta toners that are applied, in registration, during successive rotations of the photoconductive surface. The printer also generally includes a toner module with black toner since it is required in virtually all commercial color printing applications. The developed color image is then transferred from the photoconductive surface to a media sheet. Heat is usually applied to permanently fuse the image to the media sheet in order to form a completed multi-color print.
A number of factors may hinder transfer of a developed image to a media sheet and cause impaired image quality. For instance, transfer efficiency is uncertain if adhesion of the toner to the photoconductive surface is more favored energetically than adhesion of the toner to a paper surface (i.e. direct transfer), or to an intermediate transfer surface (i.e. indirect transfer). Paper sheets have a surface roughness which is dimensionally greater than toner particle size and is sometimes greater than the dimensions of the image to be imprinted. At high resolutions, very small image elements may not be successfully transferred to a paper sheet, and in extreme circumstances, may not even contact the paper. A second mode of incomplete image transfer may occur when an imaged (toned) area lacks sufficient mechanical integrity to transfer to a sheet without tearing. If parts of the image contact the sheet, but other parts do not, the image may separate. Each of these incomplete transfers may occur both in black/white and color electrophotographic printers.
The prior art includes a number of methods for improving efficiency of image transfer. In general, such methods employ a sheet of laminating material to improve transfer efficiency. The laminating material is often a thin sheet of a fusible polymer which is brought into pressure contact with the photoconductor, usually by a roller or belt. In U.S. Pat. Nos. 4,489,122 to Karorain et al, 5,060,981 to Fossum et al. and 4,968,063 to McConville et al, a laminating sheet is laid over the photoconductor prior to imaging and developing steps. The toner is directly developed onto the laminating sheet and never comes in direct contact with the photoconductive surface. In such case, the image is completely removed from the photoconductor and laminated onto a receiving material, usually paper. The transfer is accomplished directly in a single step and there is no chance for incomplete transfer unless the lamination sheet tears. A principle disadvantage of this method is that the top layer of the final image is the laminating sheet. Thus, the entire paper, including the imaged areas and the background, is coated with a polymeric, glossy, transfer sheet. This changes both the feel and appearance of the paper in ways that may not be aesthetically pleasing. It also adds to the cost per page. The electrophotographic process must also take into account that the photoconductive surface is coated with a dielectric during the charge, expose and development phases of the process. Any quality control issues associated with the thickness or electrical properties of the laminating sheet may impact negatively on print quality.
U.S. Pat. Nos. 5,106,710 to Wang et al, 5,023,668 to Kluy et al, and 5,108,865 to Zwadlo et al describe a second method for assisting transfer of a toned image from a photoconductive surface. An adhesive sheet is presented to the photoconductive surface after imaging and development. The adhesive sheet is pressed to the photoconductive surface and over the image, employing a roller or belt, with the adhesive side facing the toned image. The toner, having a greater affinity for the adhesive sheet than for the photoconductive surface, is literally pulled off the photoconductive surface.
If a single step direct transfer procedure is employed, the adhesive sheet is the layer next to the paper and the color planes appear on top of it. The adhesive sheet is generally fused to affix it to the paper. The face-up adhesive surface (which would be exposed in non-imaged areas) is also fused at this time and presumably loses its tackiness.
More often, image transfer occurs in a two step process, in which case the adhesive sheet, with color image areas adhering to it, is transferred first to an intermediate roller or belt, with the adhesive (and image) side up. It is then transferred again, adhesive side down, onto the final sheet, usually paper. After fusing, the surface may subsequently be abraded to reduce the gloss of the outer layer which is the polymeric adhesive sheet. The principle advantage of this method is that imaging is directly on the photoconductive surface and permits a different set of electrophotographic conditions than where an insulative layer intervenes between the photoconductive surface and the toner. The disadvantages of the process include increased cost per page, and as above, a different feel and look to the printed page. Further, any irregularity in the manner in which the adhesive sheet is laid against the photoconductive surface will degrade the print quality of the image.
Accordingly, it is an object of this invention to provide an improved method and system for toned image transfer from a photoconductive surface.
It is another object of this invention to provide an improved method of image transfer from a photoconductive surface which avoids the need for use of intermediate laminating sheets.
It is yet another object of this invention to provide an improved method and system for image transfer from a photoconductive surface which does not change the look and feel of the sheet which receives the image.